Orthopedic and dental implants are commonly coated with a substance to provide a surface suitable for the ingrowth of bone tissue, thereby securely anchoring the implant to the existing bone. The biocompatibility of the coating substance further minimizes implant rejection and increases the useful life of the implant. Calcium phosphate ceramics, such as tricalcium phosphate (TCP) and hydroxyapatite (HA), are particularly suitable materials. Hydroxyapatite is particularly preferred since it is a naturally occurring material in bone.
Hydroxyapatite has been applied to implants using thermal plasma spray techniques. However, hydroxyapatite disadvantageously decomposes into amorphous calcium phosphates during thermal plasma spraying. These decomposition products are more soluble in aqueous and physiological solutions than crystalline hydroxyapatite, and their presence in medical implant coatings (typically as high as 50%) increases the dissolution rate of the coating and can result in premature failure of the implant system.
Earlier attempts have been made to produce more highly crystalline hydroxyapatite coatings. A plasma sprayed coating of tricalcium phosphate (TCP) has been converted to hydroxyapatite by reaction in water at elevated temperature and atmospheric pressure; and a similar TCP coating has been converted to hydroxyapatite by hydrothermal treatment in aqueous calcium phosphate solutions at 80-200.degree. C. and 0.1-2 MPa. A mixed phase coating derived from plasma sprayed hydroxyapatite and containing amorphous calcium phosphate, tricalcium phosphate, and hydroxyapatite has been converted into hydroxyapatite by a hydrothermal treatment in pure H.sub.2 O at 200.degree. C. and 2 MPa.
None of the prior art coating processes have been able to provide a commercial hydroxyapatite coating of crystallinity greater than 90% crystalline hydroxyapatite content. Moreover, high temperature treatment in water/air environments, such as the prior art processes, result in the disadvantageous oxidation of the metallic implant ("tarnish"). Such oxidation can degrade the quality and mechanical integrity of the interface between the coating and the implant.
It is the object of the present invention to provide a hydroxyapatite coating for use on medical implant devices having improved crystallinity over prior art processes and reduced solubility of the hydroxyapatite layer.
It is another object of the present invention to provide a hydroxyapatite coated medical implant device having no disadvantageous oxidation of the metallic implant surface.